Thin-film transistor circuit and a semiconductor display using the same

ABSTRACT

To provide thin-film transistor circuits used for a driving circuit that realizes a semiconductor display capable of producing an image with high resolution and high precision without image unevenness. 
     TFTs with small channel widths are used to form an analog buffer which comprises a differential amplifier circuit and a current mirror circuit and which is used in a driving circuit of an active matrix semiconductor display. A plurality of such analog buffer circuits are connected in parallel to secure an analog buffer that has a sufficient current capacity.

This application is a is a continuation of U.S. application Ser. No.10/234,750, filed on Sep. 4, 2002 now U.S. Pat. No. 7,042,432 which is acontinuation of U.S. application Ser. No. 09/300,133, filed on Apr. 27,1999 (now U.S. Pat. No. 6,538,632 issued Mar. 25, 2003).

BACKGROUND OF THE INVENTION

The present invention relates to a driving circuit for a semiconductordisplay device using thin-film transistors. More specifically, theinvention relates to a thin-film transistor circuit which uses adifferential amplifier circuit and a current mirror circuit and which isused in a driving circuit for an active-matrix type semiconductordisplay. The invention also relates to a semiconductor display deviceusing the thin-film transistor circuits.

A technology for manufacturing thin-film transistors (TFT) using asemiconductor film formed on an inexpensive glass substrate has advancedrapidly in recent years. This is because there are growing demands foractive matrix liquid crystal displays and EL displays. The active matrixliquid crystal display has a TFT disposed in each of several tens toseveral millions of pixel areas arranged in matrix and controls electriccharges coming into or going out of individual pixel electrodes by theswitching function of the TFTs.

FIG. 11 shows a configuration of a conventional active matrix liquidcrystal display device. A shift register and a buffer circuit aregenerally called a driving circuit and are in recent years formed on thesame substrate as the active matrix circuit. Denoted 1101 is a sourcesignal line side driving circuit and 1102 a gate signal line sidedriving circuit.

Designated 1103 is an active matrix circuit (pixel matrix circuit) whichhas pixels TFT 1104 arranged in matrix. Each pixel TFT 1104 has itsdrain electrode connected to a pixel electrode. Between these pixelelectrodes and counter electrodes is sandwiched and sealed liquidcrystal. The pixel TFTs 1104 are each formed with an auxiliary capacitor1106 to hold charge.

A technology is also known which uses quartz as a substrate andmanufactures thin-film transistors using a polycrystalline silicon film.

Another technology is also known which utilizes a laser anneal method tomanufacture thin-film transistors using crystalline silicon film on aglass substrate.

In the configuration shown in FIG. 11, a timing signal from the shiftregister circuit of the source signal line side driving circuit selectsan image signal supplied to an image line, and the selected image signalis fed to the corresponding source signal line. Further, a timing signalfrom the gate signal line side driving circuit is supplied to thecorresponding gate signal line (scan line). The image signal fed to thesource signal line is written into the pixel electrode of the thin-filmtransistor selected by the timing signal from the gate signal line.

This operation is repeated by setting an appropriate timing tosuccessively write information into each pixel arranged in matrix.

After image information for one screen (one frame) has been written, thewriting of image information for the next screen is performed. In thisway, images are displayed one after another. Normally, the writing ofimage information for one screen is performed 30 or 60 times a second.

FIG. 12 shows one example of the source signal line side drivingcircuit. Reference number 1200 represents a clock input terminal, 1201 aclock line, 1202 a start pulse input terminal, 1203-1205 shiftregisters, 1206-1211 inverter type buffers, 1212 a video signal inputterminal, 1213 a video signal line, 1214-1216 and 1220-1222 switches,1217-1219 and 1225-1227 storage capacitors, 1223 a transfer signal inputterminal, 1224 a transfer signal input line, 1228-1230 analog buffers,and 1231-1233 source signal line connection terminals.

In the case of an analog gray scale, a gray scale signal entered intothe source signal line side driving circuit uses a video signal which iscontinuous in time. In the case of a normally white mode (a display modethat displays a white color when the liquid crystal is not impressedwith a voltage), a setting is made such that the displayed colorapproaches black as the absolute value of the voltage of the gradationsignal increases. To the shift registers 1203-1205, a start pulse isapplied in synchronism from the start pulse input terminal 1202, withthe video signal and are shifted by a clock pulse entered from the clockpulse line. The outputs of the shift registers 1203-1205 are fed throughthe inverter type buffers 1206-1211 to a sampling circuit.

The sampling circuit comprises switches 1214-1216 and storage capacitors1217-1219.

Here, one example of a conventional circuit used as analog buffers1228-1230 is shown in FIG. 13. Designated 1301 is a terminal connectedwith a storage capacitor and used as a signal input terminal (IN).Denoted 1302 is a terminal connected with a source signal line and usedas a signal output terminal (OUT). Reference numeral 1303 represents aconstant current source, 1304 a constant voltage source, 1305 and 1306P-channel TFTs, and 1307 and 1308 N-channel TFTs. In the analog bufferof FIG. 13, the differential circuit comprises P-channel TFTs and thecurrent mirror circuit comprises N-channel TFTs.

The operation of the analog buffer of FIG. 13 will be described. Whenthe voltage of the input terminal (IN) 1301 of the differential circuitconnected to the storage capacitor increases, the input current of thecurrent mirror circuit connected to the opposite phase output of theinput terminal (IN) 1301 decreases and the output current of the currentmirror circuit also decreases correspondingly. On the other hand, thecurrent of the same phase of the input terminal increases, causing thevoltage of the output terminal (OUT) 1302 to rise to the same voltagelevel as the input terminal of the differential circuit. Therefore, thevoltage of the source signal line connected to the output terminal (OUT)1302 becomes equal to that of the input terminal.

In recent years, as the amount of information handled increases sharply,efforts have been made to increase the display capacity and enhance theresolution of the display. Examples of computer display resolutions forsome standards are shown below in terms of pixel numbers.

Pixel number (horizontal×vertical): Standard

640×640: EGA

640×480: VGA

800×600: SVGA

1024×768: XGA

1280×1024: SXGA

Recent years have seen the spread of software even in a personalcomputer field that performs a plurality of displays which are differentin nature. This gives rise to a trend that a growing number of displaysare compatible with XGA and SXGA standards that have higher resolutionsthan VGA and SVGA.

The active matrix liquid crystal displays are very frequently used inthe field of notebook type personal computer. In recent years, they havecome to be used not only in the notebook type personal computer butoften as the displays of desktop personal computers.

In addition to the display of data signals in personal computers, theactive matrix liquid crystal displays with high resolutions have come tobe used for displaying television signals.

The buffers or analog buffers in the active matrix liquid crystaldisplay devices used for such displays are not useful if their currentcapacity is small, and thus are required to have a certain magnitude ofcurrent capacity. When buffers or analog buffers with a large currentcapacity are made using thin-film transistors, the TFTs with a largecurrent capacity, i.e., with a large channel width, are necessary.However, the TFTs with a large channel width have variations in thecrystallinity among devices, which in turn causes variations inthreshold voltage among TFTs. Hence, there are necessarily variations inthe characteristic of the buffer or analog buffer made of a plurality ofTFTs. This means that buffers or analog buffers have characteristicvariations among individual source signal lines, and thesecharacteristic variations will lead to variations in voltage applied tothe pixel matrix circuit, which in turn will cause display unevenness onthe entire display screen.

When the TFT size (channel width) is too large, in some cases, only thecentral part of TFT functions as a channel, with end portions notworking as channels, thus accelerating the deterioration of TFTs.

Further, when the TFT size is large, the heat of the TFT itselfincreases, causing variations and degradations in the threshold.

In the gate signal line side driving circuit, too, a scanning signal issuccessively supplied to the gate signal line (scanning line) accordingto the timing signal from the shift register. The digital drivingcircuit that performs a sequential line driving must drive all pixelTFTs connected to one scanning line and thus the load capacitorconnected to one scanning line is large. Therefore, the gate signal lineside driving circuit is also required to pass the timing signal from theshift register through the buffer circuit or the like to eliminatedulling. In this case also, a buffer with a large current capacity isneeded, which raises a problem described above. Particularly because thebuffer of the gate signal line is required to drive all the TFTs in thepixel matrix circuit connected to one line, the characteristicvariations will cause significant image unevenness. This is one of themost serious problems in the way to realizing high-precision,high-definition displays.

In recent years, attentions are being focused on a technology in whichthe semiconductor thin film is made polycrystalline by applying a laserbeam against the semiconductor thin film formed on a substrate. Withthis technology, it is possible to impart a high level of energy thatequals to a thermal anneal only to a desired localized area, offeringthe advantage that the whole substrate does not have to be subjected tohigh temperature.

Particularly a technology that renders a semiconductor thin filmpolycrystalline by using a pulse oscillation laser such as an excimerlaser, is drawing attention. This method throws a laser pulse of largeenergy against the semiconductor thin film to instantly melt thesemiconductor thin film, which then grows crystals as it solidifies.

There is another method which is being spotlighted. This method changesthe shape of the laser beam into a linear shape longer in width than thesubstrate to be processed and scans this beam relatively to thesubstrate. The word scan here refers to irradiating the linear laserbeam while shifting it so that the scanned paths overlap each other.

The above technique that applies a linear pulse laser beam whileshifting it and overlapping the scanned paths, however, produces linesor stripes on the surface of the laser-irradiated semiconductor thinfilm. These stripes have great adverse effects on the characteristics ofthe thin-film transistors formed on the semiconductor thin film or ondevices that will be formed on the semiconductor thin film. When analogbuffers used for the driving circuits are to be formed, in particular,their characteristics need to be uniform as mentioned above, and thusthe linear stripes pose a serious problem. In this case, within each ofthe stripes, characteristic is uniform but there are characteristicvariations among different stripes.

Even in an anneal method using a linear laser beam, the uniformity oflaser irradiation effect constitutes an issue. The high uniformityreferred to here means that similar device characteristics are obtainedwherever on the substrate the devices are formed. To enhance theuniformity is to make uniform the crystallinity of the semiconductormaterial. For enhanced uniformity, the following steps are taken.

To alleviate nonuniformity of the laser irradiation effect, it has beenfound that applying a weak pulse laser beam as a preliminary step(hereinafter referred to as a preliminary irradiation) before using astronger pulse laser beam (hereinafter referred to as a mainirradiation) improves the uniformity. This is very effective insuppressing variations and improving the characteristics of thesemiconductor device circuits.

Why the preliminary irradiation is effective in maintaining theuniformity of the film is that the film of the semiconductor materialcontaining amorphous portions described above has a property such thatits laser energy absorption factor differs significantly from those ofthe polycrystalline and single crystal films. In other words, themechanism of the two-step irradiation is that the first irradiationcrystallizes the amorphous portions remaining on the film and the secondirradiation promotes the overall crystallization. By slowly effectingthe crystallization in this way, it is possible to suppress to a certainextent the stripe-like variations produced on the semiconductor materialby the application of a linear laser beams. This countermeasuresubstantially improves the uniformity of the laser beam irradiationeffect, making the stripe pattern relatively inconspicuous.

However, when a large number (several millions to several tens ofmillions) of thin-film transistors need to be formed on a glasssubstrate for an active matrix semiconductor display, such as a liquidcrystal display, even the above-mentioned two-step laser irradiationmethod is not satisfactory in terms of the uniformity of irradiationeffect.

It is therefore an object of the present invention to solve the problemsdescribed above and to provide a thin-film transistor circuit used in adriving circuit for a semiconductor display device that can produce agood image with high precision and high resolution and without imageunevenness.

SUMMARY OF THE INVENTION

This invention provides analog buffers with a sufficiently large currentcapacity by using TFTs with a small channel width in making analogbuffers, which is used for the driving circuit of an active matrixsemiconductor display and comprises a differential amplifier circuit anda current mirror circuit, and by connecting a plurality of thesecircuits in parallel.

According to one aspect of the invention, a thin-film transistor circuitis provided which comprises n analog buffer circuits (n is a naturalnumber equal to or greater than 2) connected in parallel, each of the nanalog buffer circuits having a differential circuit and a currentmirror circuit. This achieves the objects described herein.

According to one aspect of the invention, a thin-film transistor circuitaccording to claim 1 is provided, wherein a plurality of thin-filmtransistors forming the differential circuit and a plurality ofthin-film transistors forming the current mirror circuit are arrangedwithin the same shot of a linear pulse laser beam. This achieves theobjects described herein.

According to one aspect of the invention, a thin-film transistor circuitaccording to claim 1 or 2 is provided, wherein a direction of carriermovement in the plurality of thin-film transistors is about 45 degreesto a direction of scan of the linear pulse laser beam. This achieves theobjects described herein.

According to one aspect of the invention, a thin-film transistor circuitis provided which comprises n analog buffer circuits, each of the nanalog buffer circuits having:

a differential circuit having a thin-film transistor on an input sideand m thin-film transistors on an output side, the m thin-filmtransistors having gates, sources and drains thereof at their respectivecommon voltages; and

a current mirror circuit having a thin-film transistor on the input sideand m thin-film transistors on the output side, the m thin-filmtransistors having gates, sources and drains thereof at their respectivecommon voltages. This achieves the objects described herein.

The input side thin-film transistor and the m output side thin-filmtransistors in the differential circuit of each of the n analog buffercircuits and the input side thin-film transistor and the m output sidethin-film transistors in the current mirror circuit of each of the nanalog buffer circuits may be arranged within the same shot of a linearpulse laser beam, respectively.

A direction of carrier movement in the thin-film transistors may be setat about 45 degrees to a direction of scan of the linear pulse laserbeam.

According to one aspect of the invention, a semiconductor display deviceis provided which comprises:

a pixel matrix circuit;

a source signal line side driving circuit; and

a gate signal line side driving circuit;

wherein the source signal line side driving circuit having a thin-filmtransistor circuit having n analog buffer circuits (n is a naturalnumber equal to or greater than 2) connected in parallel, each of the nanalog buffer circuits having a differential circuit and a currentmirror circuit. This achieves the objects described herein.

A plurality of thin-film transistors forming the differential circuitand a plurality of thin-film transistors forming the current mirrorcircuit may be arranged within the same shot of a linear pulse laserbeam, respectively.

A direction of carrier movement in the plurality of thin-filmtransistors may be set at about 45 degrees to a direction of scan of thelinear pulse laser beam.

According to one aspect of the invention, a semiconductor display deviceis provided which comprises:

a pixel matrix circuit;

a source signal line side driving circuit; and

a gate signal line side driving circuit;

wherein the source signal line side driving circuit having a thin-filmtransistor circuit comprising n analog buffer circuits, each of the nanalog buffer circuits having:

a differential circuit having a thin-film transistor on an input sideand m thin-film transistors on an output side, the m thin-filmtransistors having gates, sources and drains thereof at their respectivecommon voltages; and

a current mirror circuit having a thin-film transistor on the input sideand m thin-film transistors on the output side, the m thin-filmtransistors having gates, sources and drains thereof at their respectivecommon voltages. This realizes the objects described herein.

The input side thin-film transistor and the m output side thin-filmtransistors in the differential circuit of each of the n analog buffercircuits and the input side thin-film transistor and the m output sidethin-film transistors in the current mirror circuit of each of the nanalog buffer circuits may be arranged within the same shot of a linearpulse laser beam, respectively.

A direction of carrier movement in the thin-film transistors may be setat about 45 degrees to a direction of scan of the linear pulse laserbeam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a circuit configuration of an analog buffercircuit of the invention.

FIG. 2 is a diagram showing a circuit configuration of an analog buffercircuit of the invention.

FIG. 3 is a diagram showing a process of manufacturing an active matrixliquid crystal display having an analog buffer circuit of the invention.

FIG. 4 is a diagram showing a process of manufacturing an active matrixliquid crystal display having an analog buffer circuit of the invention.

FIG. 5 is a diagram showing a process of manufacturing an active matrixliquid crystal display having an analog buffer circuit of the invention.

FIG. 6 is a cross section of an active matrix liquid crystal displayhaving an analog buffer circuit of the invention.

FIG. 7 is a perspective view of an active matrix liquid crystal displayhaving an analog buffer circuit of the invention.

FIG. 8 is a diagram showing a circuit configuration of an analog buffercircuit of the invention.

FIG. 9 is a diagram showing an example arrangement of TFTs that form anactive matrix liquid crystal display having an analog buffer circuit ofthe invention.

FIG. 10 is examples of semiconductor apparatus having a semiconductordisplay of the invention.

FIG. 11 is a schematic configuration of an active matrix liquid crystaldisplay.

FIG. 12 is a schematic diagram showing an example of a source signalline side driving circuit in an active matrix liquid crystal display.

FIG. 13 is a circuitry of a conventional analog buffer.

FIG. 14 is a graph showing an applied electric potential-transmittivitycharacteristic of a thresholdless antiferroelectric mixed liquidcrystal.

FIG. 15 is examples of electronic apparatus using a display of theinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

This invention will be explained by referring to FIG. 1. FIG. 1 shows acircuit configuration of the analog buffer circuit, which is a thin-filmtransistor circuit of the invention. The analog buffer circuit of theinvention has analog buffer circuits A1-An connected in parallel (n is anatural number equal to or larger than 2). The analog buffer circuitsA1-An all have the same circuit configuration. Thus, the analog buffercircuit of the invention can be said to have a configuration comprisingn analog buffer circuits A1 connected in parallel.

The analog buffer circuit A1 has a differential circuit B1 and a currentmirror circuit C1.

The differential circuit B1 has P-channel TFTs on the input and outputsides (Tr1 (on input side) and Tr2 (on output side)). A gate electrodeof Tr1 is connected to the input terminal (IN) 101 and a gate electrodeof Tr2 is connected to the output terminal (OUT). A connecting pointbetween source or drain of Tr1 and source or drain of Tr2 is connectedto a constant current source 103. The gate electrode of Tr2 is connectedto either source or drain, that is the other one of the source or drainconnected to the constant current source 103.

The current mirror circuit C1 has N-channel TFTs on the input and outputsides (Tr3 (on input side) and Tr4 (on output side)). A connecting pointbetween source or drain of Tr3 and source or drain of Tr4 is connectedto a constant voltage source 106. A gate electrode of Tr3 and a gateelectrode of Tr4 are connected together. The gate electrodes of Tr3 andTr4 are connected to either source or drain of Tr3 which is the otherone of the source or drain connected to the source or drain of Tr4 (orconnected to the constant voltage source 106).

The other one of source or drain of Tr1 connected to the constantcurrent source 103 is connected to the other one of source or drain ofTr3 connected to Tr4 (or connected to the constant voltage source 106).The other one of source or drain of Tr2 connected to the constantcurrent source 103, is connected to the other one of source or drain ofTr4 connected to Tr3 (or connected to the constant voltage source 106).In this way, the differential circuit B1 and the current mirror circuitC1 are connected.

An analog buffer circuit A2 has a differential circuit B2 and a currentmirror circuit C2 in the same way as does the analog buffer circuit A1.

The differential circuit B2 has P-channel TFTs on the input and outputsides (Tr5 (on input side) and Tr6 (on output side)). A gate electrodeof Tr5 is connected to the input terminal (IN) 101 and a gate electrodeof Tr6 is connected to the output terminal (OUT) 102. A connecting pointbetween source or drain of Tr5 and the source or drain of Tr6 isconnected to a constant current source 104. The gate electrode of Tr6 isconnected to its source or drain, which is opposite its source or drainconnected to the constant current source 104.

The current mirror circuit C2 has N-channel TFTs on the input and outputsides (Tr7 (on input side) and Tr8 (on output side)). A connecting pointbetween source or drain of Tr7 and source or drain of Tr8 is connectedto a constant voltage source 106. A gate electrode of Tr7 and a gateelectrode of Tr8 are connected together. The gate electrodes of Tr7 andTr8 are connected to the other one of source or drain of Tr7 connectedto Tr8.

The other one of drain or source of Tr5 connected to the constantcurrent source 104, is connected to the other one of source or drain ofTr7 connected to Tr8 (or connected to the constant voltage source 106).The other one of source or drain of Tr6 connected to the constantcurrent source 104, is connected to the other one of source or drain ofTr8 connected to the source or drain of Tr7 (or connected to theconstant voltage source 106). In this way, the differential circuit B2and the current mirror circuit C2 are connected.

An analog buffer circuit An has a differential circuit Bn and a currentmirror circuit Cn in the same way as do the analog buffer circuits A1,A2.

The differential circuit Bn has P-channel TFTs on the input and outputsides (Tr4 n-3 (on input side) and Tr4 n-2 (on output side)). A gateelectrode of Tr4 n-3 is connected to the input terminal (IN) 101 and agate electrode of Tr4 n-2 is connected to the output terminal (OUT) 102.A connecting point between source or drain of Tr4 n-3 and source ordrain of Tr4 n-2 is connected to a constant current source 105. The gateelectrode of Tr4 n-2 is connected to its source or drain, which isopposite its source or drain connected to the constant current source105.

The current mirror circuit Cn has N-channel TFTs on the input and outputsides (Tr4 n-1 (on input side) and Tr4 n (on output side)). A connectingpoint between the source or drain of Tr4 n-1 and the source or drain ofTr4 n is connected to a constant voltage source 106. A gate electrode ofTr4 n-1 and a gate electrode of Tr4 n are connected together. The gateelectrodes of Tr4 n-1 and Tr4 n are connected to the other one of sourceor drain of Tr4 n-1 that is connected to the source or drain of Tr4 n(or connected to the constant voltage source 106).

The other one of source or drain of Tr4 n-3 connected to the constantcurrent source 105, is connected to the other one of source or drain ofTr4 n-1 connected to Tr4 n (or connected to the constant voltage source106). The other one of source or drain of Tr4 n-2 connected to theconstant current source 105, is connected to the other one of source ordrain of Tr4 n connected to Tr4 n-1 (or connected to the constantvoltage source 106). In this way, the differential circuit Bn and thecurrent mirror circuit Cn are connected.

The circuit configurations of analog buffers A3 to An−1 (not shown) arealso similar to those of the analog buffers A1, A2 and An.

As described above, the analog buffer circuit of the invention comprisesn analog buffer circuits connected in parallel. As for the TFTs makingup the analog buffer circuit of the invention, those with a relativelysmall channel width (100 μm or less (preferably 90 μm or less)), ratherthan those with a large channel width, are used. This can preventcharacteristic variations of TFTs and thus reduce characteristicvariations of the analog buffer circuit. Furthermore, this can realizean analog buffer circuit with a large current capacity.

The polarities of the thin-film transistors used in the differentialcircuit and the current mirror circuit are opposite. In FIG. 1, thedifferential circuit uses P-channel thin-film transistors and thecurrent mirror circuit uses N-channel thin-film transistors. Thepolarities of P- and N-channel transistors may be reversed. It is noted,however, that they do not operate if their polarities are the same.

When the active matrix liquid crystal display using the analog buffercircuit of the invention is to be fabricated by the laser technologymentioned earlier, it is possible to further reduce the characteristicvariations of the analog buffer circuit as described below.

When the analog buffer of the invention shown in FIG. 1 is used, thedifferential circuits B1-Bn are arranged so that they are included inthe same pulse of the linear laser beam, and the current mirror circuitsC1-Cn are arranged likewise to be included in the same pulse of thelinear laser beam. The semiconductor film made polycrystalline by thesame pulse of the linear laser beam is known to have smallcharacteristic variations. With this arrangement, the characteristicvariations of the analog buffer circuit can be minimized further.Furthermore, both the differential circuits B1-Bn and the current mirrorcircuits C1-Cn may be arranged to be included in the same pulse of thelinear laser beam.

Further it is also possible to arrange (pattern) the analog buffercircuit so that direction of active layer of TFTs making up the analogbuffer circuit, or the direction of movement of carriers, is inclined tothe linear laser beam scan direction, i.e., the direction perpendicularto the line.

When the analog buffer circuit is arranged so that the TFT active layeris inclined to the scan direction of the linear laser beam, it is knownthat the characteristic variations of individual TFTs decrease. Hence,this arrangement can further reduce variations of the analog buffercharacteristic that directly affects the image quality.

The invention will be described in detail in conjunction with thefollowing embodiments. It should be noted that the following embodimentsare only example implementations of the invention and that the inventionis not limited to these embodiments

Embodiment 1

In this embodiment, a case where the analog buffer of the invention isapplied to the source signal line side driving circuit of an activematrix liquid crystal display will be described. The analog buffer ofthe invention can also be used for the gate signal line side drivingcircuit. The number of pixels in the active matrix liquid crystaldisplay of this embodiment is 1024 horizontal×768 vertical.

FIG. 2 will be referred. FIG. 2 shows a circuit configuration of theanalog buffer circuit of this embodiment. The analog buffer circuit ofthis embodiment has three analog buffer circuits A1-A3 connected inparallel (n=3). The analog buffer circuits A1-A3 each has a differentialcircuit and a current mirror circuit. The analog buffer circuits A1-A3are similar to the above-described circuit configuration having n analogbuffer circuits connected in parallel, in that each of the analog buffercircuits is connected to an input terminal (IN) 201 and an outputterminal (OUT) 202 and also to constant current sources 203-205 and aconstant voltage source 206.

The analog buffer circuit A1 has a differential circuit B1 and a currentmirror circuit C1. P-channel TFTs (Tr1 and Tr2) making up thedifferential circuit B1 have a channel width of 30 μm and N-channel TFTs(Tr3 and Tr4) forming the current mirror circuit C1 have a channel widthof 30 μm.

As to the analog buffer circuit A2 and analog buffer circuit A3, thesame also holds. The P-channel TFTs forming these circuits (Tr5, Tr6,Tr9 and Tr10) have a channel width of 30 m and the N-channel TFTs (Tr7,Tr8, Tr11 and Tr12) have a channel width of 30 μm.

While in this embodiment the number of TFTs forming the differentialcircuit is set equal to that of the TFTs forming the current mirrorcircuit, they do not have to be the same. Although the channel widths ofthe P-channel TFTs or N-channel TFTs are set to 30 m, they need only tobe set equal to or smaller than 100 μm (preferably equal to or smallerthan 90 μm).

In this embodiment, the analog buffer circuit is made by connecting inparallel three analog buffer circuits each comprising TFTs withrelatively small channel widths. Hence, compared with a case where theanalog buffer circuit is formed of only one analog buffer circuit, thesize of a plurality of TFTs (channel width) making up each of the threeanalog buffer circuits needs only to be one-third. Thus, the arrangementof this embodiment can minimize the self-heating of TFTs and thereforeprevent change or degradation of threshold values of the TFTs whichwould otherwise occur due to self-heating. If one or two of the threeanalog buffer circuits A1-A3 forming the analog buffer circuit shouldfail, the remaining two or one analog buffer circuit remainsoperational, thereby improving yields of the product.

By using a plurality of TFTs with small channel widths to form theanalog buffer which directly affects the image quality of the activematrix liquid crystal display and by securing a necessary capacity, itis possible to reduce characteristic variations and self-heating,enhance reliability and minimize deterioration.

One example method of manufacturing an active matrix liquid crystaldisplay having the analog buffer of this embodiment in a driving circuitwill be explained. The manufacturing method described below is only oneexample to implement the present invention, and other manufacturingmethods may be used to realize the active matrix liquid crystal displayhaving thin-film transistor circuits of the invention.

Here, an example case of forming a plurality of TFTs on a substratehaving an insulated surface to form a pixel matrix circuit, a drivingcircuit, a logic circuit, etc. in a monolithic manner is shown in FIGS.3 to 6. This embodiment shows how one pixel of the pixel matrix circuitand a CMOS circuit, which is a basic circuit of other circuits (such asa driving circuit and a logic circuit both having the analog buffers ofthe invention) are formed simultaneously on the same substrate. Further,although this embodiment describes the manufacturing process for a casewhere the P-channel TFT and the N-channel TFT each have one gateelectrode, the similar process can also be applied to the CMOS circuitmade of TFTs having a plurality of gate electrodes, such as double gateor triple gate type TFTs.

Referring to FIG. 3, a quartz substrate 301 is prepared as a substratehaving an insulated surface. It is also possible to use a siliconsubstrate formed with a thermal oxide film, rather than the quartzsubstrate. Further, another method may be used which temporarily formsan amorphous silicon film over the quartz substrate and thermallyoxidizes it completely to form an insulating film. It is also possibleto use a quartz substrate or ceramics substrate formed with a siliconnitride film as an insulating film.

An amorphous silicon film 302 is formed over the substrate 301 by a lowpressure CVD method, plasma CVD method or sputtering method. Theamorphous silicon film 302 is adjusted so that its final thickness (athickness considering a thickness reduction after thermal oxidation)will be 10-100 nm (preferably 30-60 nm). It is important during the filmforming process to perform a comprehensive control on concentrations ofimpurities in the film.

While in this embodiment the amorphous silicon film 302 is formed overthe substrate 301, other semiconductor films may be used instead of theamorphous silicon film. For example, a compound of silicon andgermanium, expressed as Si_(X)Ge_(1-X) (0<X<1), may be used.

In this embodiment, the process is controlled so that the concentrationsof C (carbon) and N (nitrogen)—impurities that hinder crystallization—inthe amorphous silicon film 302 are both less than 5×10¹⁸ atoms/cm³(typically, 5×10¹⁷ atoms/cm³ or less or preferably 2×10¹⁷ atoms/cm³ orless) and that the concentration of O (oxygen) is less than 1.5×10¹⁹atoms/cm³ (typically, 1×10¹⁸ atoms/cm³ or less or preferably 5×10¹⁷atoms/cm³). This is because, if these impurities exist at concentrationshigher than these levels, they will adversely affect the subsequentcrystallization process and degrade the film quality aftercrystallization. In this specification, the in-film impurityconcentrations of these elements described above are defined by minimumvalues of measurements taken by SIMS (secondary ion mass spectrometry).

To realize the above requirements, the low pressure thermal CVD furnaceused in this embodiment should preferably be dry-cleaned periodically tokeep the film depositing chamber clean. The dry cleaning is done byfeeding 100-300 sccm of ClF₃ (chlorine fluoride) gas into the furnaceheated to about 200-400° C. so that the film depositing chamber iscleaned with a fluorine gas generated by thermal decomposition.

According to the findings obtained by the applicant of this invention,when the in-furnace temperature is set at 300° C. and 300 sccm of ClF₃(chlorine fluoride) gas is supplied, an adhering material about 2 mmthick (mainly composed of silicon) can be removed completely in fourhours.

The hydrogen concentration in the amorphous silicon film 302 is a veryimportant parameter. It seems that a film with good crystallinity can beobtained by keeping the hydrogen content low. It is therefore preferredthat the amorphous silicon film 302 is formed by the low pressurethermal CVD. The plasma CVD may also be used by optimizing the filmdepositing condition.

During the process of forming the amorphous silicon film 302, it iseffective to add an impurity element (13-group elements, typicallyboron, or 15-group elements, typically phosphorus) to control thethreshold voltage (Vth) of TFTs. The amount to be added should bedetermined by considering the Vth obtained when the Vth controllingimpurities are not added.

Next, the process of crystallizing the amorphous silicon film 302 isperformed. As crystallization means the technology disclosed by JapanesePatent Application Laid-Open Heisei 7-130652 is used. Either anembodiment 1 or an embodiment 2 in the official gazette may be used. Itis preferred in this embodiment, however, to use the technologydisclosed by the embodiment 2 (which is detailed in Japanese PatentApplication Laid-Open Heisei 8-78329). U.S. Pat. No. 5,643,826corresponds to the Japanese Patent Application Laid-Open Heisei7-130652. The contents disclosed in these Japanese patent applicationsand U.S. patent are incorporated herein by reference.

The technology described in Japanese Patent Application Laid-Open Heisei8-78329 first forms a mask insulation film 303 that selects areas towhich a catalytic element is to be added. The mask insulation film 303has a plurality of openings for adding the catalytic element. Thepositions of the openings determine the positions of crystal areas.

Then, a solution containing nickel (Ni) as a catalytic element thatpromotes crystallization of amorphous silicon film is applied by spincoating to form a Ni containing layer 304. In addition to nickel, thecatalytic element may include cobalt (Co), iron (Fe), palladium (Pd),germanium (Ge), platinum (Pt), copper (Cu), gold (Au), etc. (FIG. 3A).

The process of adding the catalytic element may use ion implantationusing a resist mask or plasma doping. In this case, the reduction of thedoped areas and the control of growth distance of the horizontal growthareas become easy. Hence, this is an effective technology when formingminiaturized circuits.

Next, after the catalytic element adding process is finished, hydrogenis removed at 500° C. for about 2 hours. After this, under an inertatmosphere, hydrogen atmosphere or oxygen atmosphere the substrate isheated at a temperature of 500-700° C. (typically 550-650° C. orpreferably 570° C.) for 4-24 hours to crystallize the amorphous siliconfilm 302. In this embodiment, the heating process is performed at 570°C. in a nitrogen atmosphere for 14 hours.

At this time, the crystallization of the amorphous silicon film 302begins preferentially from nuclei that are generated in the regions 305and 306 doped with nickel, thus forming crystal areas 307 and 308 thathave grown almost in parallel with the surface of the substrate 301.These crystal areas 307 and 308 are called lateral growth areas. Becausethe lateral growth areas have an aggregate of crystals scatteredrelatively uniformly, it has an advantage of excellent overallcrystallinity (FIG. 3B).

When the technology described in the embodiment 1 of the Japanese PatentApplication Laid-Open Heisei 7-130652 is used, an area that can becalled a lateral growth area is also formed microscopically. However,nucleus generation occurs nonuniformly in the plane and thus there is aproblem in terms of controlling the grain boundary.

After the heating process for crystallization is complete, the maskinsulation film 303 is removed and a patterning process is performed toform island-shaped semiconductor layers (active layers) 309, 310, 311made of the lateral growth areas 307 and 308 (FIG. 3C).

Denoted 309 is an active layer of an N-channel TFT forming a CMOScircuit, 310 an active layer of a P-channel TFT forming a CMOS circuit,and 311 an active layer of an N-channel TFT (pixel TFT) forming a pixelmatrix circuit.

After the active layers 309, 310, 311 have been formed, a gateinsulation film 312 containing silicon is formed over the active layers(FIG. 3C).

Then, as shown in FIG. 3D, the substrate is heated to remove or reducethe catalytic element (nickel) (a catalytic element gettering process).This heating process mixes a halogen element in the processingatmosphere to utilize the gettering effect of the halogen element on themetal element.

To obtain a sufficient gettering effect of the halogen element, theheating process should preferably be performed at a temperature inexcess of 700° C. Below this temperature, decomposition of halogencompounds in the processing atmosphere becomes difficult, and that maymake it impossible to obtain a desired gettering effect.

In this embodiment, therefore, the heating process is performed at atemperature in excess of 700° C., preferably 800-1000° C. (typically950° C.), for 0.1-6 hours, typically 0.5-1 hour.

In this embodiment, an example case of heating the substrate at 950° C.for 30 minutes in an oxygen atmosphere containing 0.5-10 vol % ofhydrogen chloride (in this embodiment 3 vol %) will be explained.Setting the HCl density higher than this level is not desirable becauseit forms unevenness on the surface of about the film thickness of theactive layers 309, 310, 311.

Although HCl gas is used as a compound containing halogen elements,other gases may be used. For example, it is possible to use one or moreof halogen containing compounds such as HF, NF₃, HBr, Cl2, ClF₃, BCl₂,F₂ and Br₂.

In this process, nickel in the active layers 309, 310, 311 is consideredto be removed by the gettering action of chlorine and released into anopen air in the form of volatile nickel chloride. This process reducesthe nickel concentration in the active layers 309, 310, 311 below 5×10¹⁷atoms/cm³.

The value of 5×10¹⁷ atoms/cm³ is the lower limit of detection by theSIMS (secondary ion mass spectrometry). The analysis of TFTs fabricatedby the applicant of this invention shows that no nickel effect on theTFT characteristic was observed for the nickel concentration of 1×10¹⁸atoms/cm³ or lower (preferably 5×10¹⁷ atoms/cm³ or lower). However, theimpurity concentrations in this specification are defined by the minimumvalues of measurements taken by the SIMS analysis.

At the interface between the active layers 309, 310, 311 and the gateinsulation film 312, a thermal oxidation reaction proceeds by theheating process described above, increasing the thickness of the gateinsulation film 312 by a thickness of the thermally oxidized film.Forming the thermally oxidized film in this way can produce asemiconductor/insulation film interface with a very low interface state.It also has an effect of preventing failure of formation of thermallyoxidized film at the end of the active layer (edge thinning).

It may be effective to perform the catalytic element gettering processafter removing the mask insulation film 303 before patterning the activelayer. The catalytic element gettering process may also be performedafter the active layer is patterned. It is also possible to combine anyof these gettering processes.

The catalytic element gettering process can be performed by using P(phosphorus). The gettering process using phosphorus may be combinedwith the above-mentioned gettering process. It is also possible to useonly the gettering process using phosphorus.

Further, performing the heating process at 950° C. for about 1 hour in anitrogen atmosphere following the heating process in the halogenatmosphere is also effective in improving the film quality of the gateinsulation film 312.

The SIMS analysis has found that the halogen elements that were used forthe gettering process remain in the active layers 309, 310, 311 at aconcentration of 1×10¹⁵ atoms/cm³ to 1×10²⁰ atoms/cm³. It is alsoconfirmed by the SIMS analysis that the halogen elements are distributedat high concentrations between the active layers 309, 310, 311 and thethermally oxidized film formed by the heating process.

The SIMS analysis has also found that the concentrations ofrepresentative impurities, C (carbon), N (nitrogen), O (oxygen) and S(sulfur), are below 5×10¹⁸ atoms/cm³ (typically 1×10¹⁸ atoms/cm³ orlower).

The lateral growth areas of the active layers obtained in this mannerexhibit a peculiar crystal structure consisting of an aggregate ofbar-like or flat bar-like crystals. The features of this peculiarcrystal structure will be described later.

Next, let us turn to FIG. 4. First, a metal film not shown which is mademainly of aluminum is formed, and then prototypes of gate electrodes313, 314, 315 are formed by patterning. In this embodiment an aluminumfilm containing 2 wt % of scandium is used (FIG. 4A).

Instead of the aluminum film containing 2 wt % of scandium, apolysilicon film injected with impurities may be used as gateelectrodes.

Next, by using a technology described in Japanese Patent ApplicationLaid-Open Heisei 7-135318, porous anode oxide films 316, 317, 318,nonporous anode oxide films 319, 320, 321, and gate electrodes 322, 323,324 are formed (FIG. 4B).

After the state of FIG. 4B is obtained, the gate insulation film 312 isetched away, with the gate electrodes 322, 323, 324 and porous anodeoxide films 316, 317, 318 used as masks. Then, the porous anode oxidefilms 316, 317, 318 are removed to obtain the state of FIG. 4C. In FIG.4C, denoted 325, 326 and 327 are gate insulation films after theprocess.

Next, an impurity element for giving a conductivity is added. Theimpurity element may include P (phosphorus) or As (arsenic) forN-channel type and B (boron) or Ga (gallium) for P-channel type.

In this embodiment, the impurity injection processes for forming theN-channel and P-channel TFTs are each performed in two steps.

First, an impurity is added for forming the N-channel TFTs. A firstimpurity injection (using P (phosphorus) in this embodiment) isperformed at a high acceleration voltage of about 80 keV to form an n−region. An adjustment is made of the impurity injection so that theP-ion concentration in the n− region will be 1×10¹⁸ atoms cm³ to 1×10¹⁹atoms/cm³.

A second impurity injection is carried out at a low acceleration voltageof about 10 keV to form an n+ region. Because the acceleration voltageis low, the gate insulation film works as a mask. An adjustment is madeso that a sheet resistance of the n+ region is 500Ω or less (preferably300Ω or less).

With the above steps taken, a source region 328, a drain region 329, lowconcentration impurity regions 330, and a channel formation region 331of an N-channel TFT of the CMOS circuit are formed. Further, a sourceregion 332, a drain region 333, low concentration impurity regions 334,and a channel formation region 335 of an N-channel TFT forming a pixelTFT are defined (FIG. 4D).

In the state shown in FIG. 4D, the active layer of the P-channel TFTforming the CMOS circuit has the same structure as the active layer ofthe N-channel TFT.

Next, as shown in FIG. 5A, a resist mask 336 is deposited over theN-channel TFTs and impurity ions for giving P-type conductivity areinjected (in this embodiment boron is used).

This process is also performed in two steps as in the preceding impurityinjection process. In this case, because the N-channel type must beinverted into the P-channel type, B (boron) is added in a concentrationseveral times higher than that of the P (phosphorus) ions.

Thus, a source region 337, a drain region 338, low concentrationimpurity regions 339 and a channel formation region 340 of a P-channelTFT of a CMOS circuit are formed (FIG. 5A).

After the active layer is completed as described above, the impurityions are activated by a combination of furnace anneal, laser anneal,lamp anneal, etc. At the same time, damages in the active layersustained during the injection process are repaired.

Next, a laminated films consisting of a silicon oxide film and a siliconnitride film are formed as an inter-layer insulation film 341. Aftercontact holes are formed, source electrodes 342, 343, 344 and drainelectrodes 345, 346 are formed to obtain the state shown in FIG. 5B. Anorganic resin film may also be used as the inter-layer insulation film341.

After the state shown in FIG. 5B is obtained, a first inter-layerinsulation film 347 made of organic resin film is formed to a thicknessof 0.5-3 μm. The organic resin film may include polyimide, acrylics andpolyimide amide. Among the advantages of the organic resin film are theease with which the film can be made, the ability to increase the filmthickness easily, the ability to reduce parasitic capacitance because ofits low relative permittivity, and an excellent planarity. Other organicresin films than mentioned above may also be used.

Next, a black matrix 348 made of a light shielding film is formed overthe first inter-layer insulation film 347 to a thickness of 100 nm. Inthis embodiment, a titanium film is used as the black matrix 348 but aresin film containing black pigment may be used.

When a titanium film is used for the black matrix 348, a part of thedriving circuit and other peripheral circuits can be formed of titanium.The titanium wiring is formed simultaneously with the black matrix 348.

After the black matrix 348 is formed, one of a silicon oxide film, asilicon nitride film and an organic resin film or a laminated film ofthese is formed as a second inter-layer insulation film 349 to athickness of 0.1-0.3 μm. Then, contact holes are formed in theinter-layer insulation film 347 and the inter-layer insulation film 349.A pixel electrode 350 is formed to a thickness of 120 nm. In theconstituent of this embodiment, an auxiliary capacitor is formed inareas where the black matrix 348 and the pixel electrode 350 overlapeach other (FIG. 5C). This embodiment is an example of a transmissiontype active matrix liquid crystal display and thus a transparentconductive film such as ITO is used as a conductive film that forms thepixel electrode 350.

Next, the substrate as a whole is heated at 350° C. in a hydrogenatmosphere for 1-2 hours to hydrogenate the entire device and therebycompensate for dangling bonds in a film (specially in an active layer).With these processes performed, the CMOS circuit and the pixel matrixcircuit can be fabricated on the same substrate.

Next, by referring to FIG. 6, the process of manufacturing an activematrix liquid crystal display using the active matrix substratefabricated as described above will be explained.

An alignment film 351 is formed over the active matrix substrate in thestate of FIG. 5C. This embodiment uses polyimide for the alignment film351. Next, a counter substrate is prepared. The counter substratecomprises a glass substrate 352, a transparent conductive film 353 andan alignment film 354.

This embodiment uses as an alignment film such a polyimide film as willorientate the liquid crystal molecules parallel to the substrate. Afterthe alignment film is formed, a rubbing process is performed toorientate liquid crystal molecules in parallel at a predeterminedpre-tilt angle.

Next, the active matrix substrate and the counter substrate, which haveundergone the above processes, are bonded together with seal members andspacers (neither of them shown) interposed, by the known cell assemblyprocess. Then, a liquid crystal material 355 is injected between the twosubstrates and completely sealed by a sealant (not shown). Thus, atransmissive type active matrix liquid crystal display as shown in FIG.6 is completed.

FIG. 7 shows a perspective view of a completed active matrix liquidcrystal display. Reference number 701 represents an active matrixsubstrate, 702 a pixel matrix circuit, 703 a source signal line sidedriving circuit, 704 a gate signal line side driving circuit, 705 otherperipheral circuits, and 706 a counter substrate. As shown in FIG. 7,the active matrix liquid crystal display device of this embodiment hasprojected to the outside only one end face of the active matrixsubstrate to which FPC is attached, with the remaining three end facesaligned.

In this embodiment, the liquid crystal display performs display in a TN(twisted nematic) mode. Hence, a pair of polarizers (not shown) aredisposed in a crossed-Nicols arrangement (a pair of polarizers arearranged with their polarizing axes crossing each other at right angles)and hold the liquid crystal panel therebetween.

Thus, when not applied with a voltage, it is understood that the liquidcrystal display of this embodiment shows a white color, i.e., itperforms an image display in a so-called normally white mode.

With the manufacturing method described above, it is understood that theactive matrix liquid crystal display device of this embodiment has thedriving circuit, other peripheral devices and pixels integrally formedover an insulation substrate such as quartz substrate and glasssubstrate.

When the polycrystallization of the semiconductor thin film is done by alinear laser beam, this manufacturing method arranges the differentialcircuits B1-B3 in a manner to include in the same pulse of the linearlaser and also arranges the current mirror circuits C1-C3 in a manner toinclude in the same pulse of the linear laser beam. The semiconductorthin film that is rendered polycrystalline by the same pulse of thelinear laser is known to have small characteristic variations. With thismethod the characteristic variations of the analog buffer circuit can beminimized further. It is also possible to have both the differentialcircuits B1-B3 and the current mirror circuits C1-C3 included in thesame pulse of the linear laser beam.

Embodiment 2

FIG. 8 shows another embodiment of the invention. FIG. 8 represents acase where the constituent of this invention is used for the analogbuffer circuit of the source signal line side driving circuit of theactive matrix semiconductor display. In this embodiment, the number ofthin-film transistors on the output side of the differential circuitsand the current mirror circuits is increased by m times (m is a naturalnumber equal to or larger than 2). In other words, the number ofcircuits on the output side of the differential circuits and the currentmirror circuits is multiplied by m. This is done to enhance the currentcapability of the analog buffer circuit.

The analog buffer circuit of this embodiment has n analog buffercircuits A1-An (n is a natural number equal to or larger than 2). Thiscircuit is identical to the previous circuit configuration in that nanalog buffer circuits are connected in parallel.

An analog buffer circuit A1 has a differential circuit B1 and a currentmirror circuit C1.

The differential circuit B1 has a P-channel TFT (Tr1) on the input sideand P-channel TFTs (Tr2,1, Tr2,2,

, Tr2,m) on the output side. Gate electrodes of Tr2,1, Tr2,2,

, Tr2,m are connected so that they are at the same electric potential. Agate electrode of Tr1 is connected to an input terminal (IN) 801 andgate electrodes of Tr2,1, Tr2,2,

, Tr2,m are connected to an output terminal (OUT) 802. Sources or drainsof Tr2,1, Tr2,2, . . . , Tr2,m are connected respectively so that theyare at the same electric potential. A connecting point between thesource or drain of Tr1 and the sources or drains of Tr2,1, Tr2,2, . . ., Tr2,m is connected to a constant current source 803. The gateelectrodes of Tr2,1, Tr2,2, . . . , Tr2,m are connected to their sourcesor drains, which are opposite their sources or drains connected with theconstant current source 803.

The current mirror circuit C1 has an N-channel TFT (Tr3) on the inputside and N-channel TFTs (Tr4,1, Tr4,2,

, Tr4,m) on the output side. The source or drain of Tr3 and the sourcesor drains of Tr4,1, Tr4,2, . . . , Tr4,m are connected together, andtheir connecting point is connected to a constant electric potentialsource 806. A gate electrode of Tr3 and gate electrodes of Tr4,1, Tr4,2,. . . , Tr4,m are connected together. The gate electrode of Tr3 and thegate electrodes of Tr4,1, Tr4,2, . . . , Tr4,m are connected to thesource or drain of Tr3, which is opposite its source or drain connectedwith the sources or drains of Tr4,1, Tr4,2, . . . , Tr4,m (or connectedwith the constant electric potential source 806).

The source or drain of Tr1, which is opposite its source or drainconnected with the constant current source 803, is connected to thesource or drain of Tr3, which is opposite to the source or drainconnected with the sources or drains of Tr4,1, Tr4,2, . . . , Tr4,m (orconnected with the constant electric potential source 806). The sourcesor drains of Tr2,1, Tr2,2, . . . , Tr2,m, which are opposite theirsources or drains connected with the constant current source 803, areconnected with sources or drains of Tr4,1, Tr4,2, . . . , Tr4,m, whichare opposite their sources or drains connected with the source or drainof Tr3 (or connected with the constant electric potential source 806).The differential circuit B1 and the current mirror circuit C1 areconnected in this manner.

Like the analog buffer circuit A1, the analog buffer circuits A2,

, An (A3 to An−1 are not shown) each have a differential circuit and acurrent mirror circuit.

The analog buffer circuit An has a differential circuit Bn and a currentmirror circuit Cn.

The differential circuit Bn has a P-channel TFT (Tr4 n-3) on the inputside and P-channel TFTs (Tr4 n-2,1, Tr4 n-2,2,

, Tr4 n-2,m) on the output side. Gate electrodes of Tr4 n-2,1, Tr4n-2,2, . . . , Tr4 n-2,m are all connected so that they are at the sameelectric potential. A gate electrode of Tr4 n-3 is connected to an inputterminal (IN) 801 and gate electrodes of Tr4 n-2,1, Tr4 n-2,2, . . . ,Tr4 n-2,m are connected to an output terminal (OUT) 802. Sources ordrains of Tr4 n-2,1, Tr4 n-2,2, . . . , Tr4 n-2,m are connected so thatthey are at the same electric potential. A connecting point between thesource or drain of Tr4 n-3 and the sources or drains of Tr4 n-2,1, Tr4n-2,2, . . . , Tr4 n-2,m is connected to a constant current source 805.The gate electrodes of Tr4 n-2,1, Tr4 n-2,2, . . . , Tr4 n-2,m areconnected to their sources or drains, which are opposite their sourcesor drains connected with the constant current source 805.

The current mirror circuit Cn has an N-channel TFT (Tr4 n-1) on theinput side and N-channel TFTs (Tr4 n,1, Tr4 n,2, . . . , Tr4 n,m) on theoutput side. The source or drain of Tr4 n-1 and the sources or drains ofTr4 n,1, Tr4 n,2, . . . , Tr4 n,m are connected together, and theirconnecting point is connected to the constant electric potential source806. The gate electrode of Tr4 n-1 and the gate electrodes of Tr4 n,1,Tr4 n,2, . . . , Tr4 n,m are connected together. The gate electrode ofTr4 n-1 and the gate electrodes of Tr4 n,1, Tr4 n,2, . . . , Tr4 n,m areconnected to the source or drain of Tr4 n-1, which is opposite itssource or drain connected to the sources or drains of Tr4 n,1, Tr4 n,2,. . . , Tr4 n,m (or connected to the constant electric potential source806).

The source or drain of Tr4 n-3, which is opposite its source or drainconnected with the constant current source 805, is connected to thesource or drain of Tr4 n-1, which is opposite its source or drainconnected with the sources or drains of Tr4 n,1, Tr4 n,2, . . . , Tr4n,m (or connected with constant electric potential source 806). Thesources or drains of Tr4 n-2,1, Tr4 n-2,2, . . . , Tr4 n-2,m, which areopposite their sources or drains connected with the constant currentsource 805, are connected to the sources or drains of Tr4 n,1, Tr4 n,2,. . . , Tr4 n,m, which are opposite their sources or drains connectedwith Tr4 n-1 (or connected with the constant electric potential source806). The differential circuit Bn and the current mirror circuit Cn areconnected in this manner.

The circuit configurations of the analog buffers A2 to An−1 (neithershown) are similar to that of the analog buffer A1 or An.

One side of all constant current sources is connected to the constantelectric potential source 806 as shown.

In this embodiment the channel width of the P-channel TFTs forming thedifferential amplifier is 30 μm. The channel width of the N-channel TFTsforming the current mirror circuit is 30 μm. While the channel widths ofindividual TFTs in this embodiment are set at 30 μm, they need only tobe 100 μm or less (preferably 90 μm or less).

The operation of the analog buffer circuit of this embodiment will bedescribed. When the electric potential of the input terminal falls, mostof the current of the constant current source flows into the P-channelTFTs of the differential circuit on the input side and into the input ofthe current mirror circuit. The number of TFTs on the output side of thecurrent mirror circuit is n times that on the input side, so that acurrent n times the constant current source can be drawn in from theoutput terminal, which in turn allows the source signal line connectedto the output terminal to be driven at high speed. When the electricpotential at the input terminal is almost equal to that at the outputterminal, the current of the constant current source is shunted to theoutput side TFTs and the input side TFTs. In this case, because theratio in the number of TFTs in the differential circuit between theoutput side and the input side is 1:n and because the ratio in thenumber of TFTs in the current mirror circuit between the output side andthe input side is 1:n, the electric potential of the input terminal andthe output terminal are not affected by the number of TFTs.

The polarities of the thin-film transistors used in the differentialcircuit are opposite to those used in the current mirror circuit. Whilein this embodiment the differential circuit uses P-channel thin-filmtransistors and the current mirror circuit uses N-channel thin-filmtransistors, their polarities may be reversed but they do not operate iftheir polarities are the same.

The active matrix liquid crystal display having the analog buffercircuit of this embodiment can be manufactured according to themanufacturing method of the embodiment 1. The manufacturing method isnot limited to that of the embodiment 1.

In the manufacturing method, when the semiconductor film is to be madepolycrystalline by a linear laser beam and the analog buffer of theinvention is used, the differential circuits B1-Bn are arranged so thatthey are included in the same pulse of the linear laser beam and alsothe current mirror circuits C1-Cn are arranged so that they are includedin the same pulse of the linear laser beam. The semiconductor film madepolycrystalline by the same pulse of the linear laser pulse is known tohave small characteristic variations. This arrangement can furtherminimize the characteristic variations of the analog buffer circuit.Further, it is also possible to arrange both the differential circuitsB1-Bn and the current mirror circuits C1-Cn so that they are included inthe same pulse of the linear laser beam.

Embodiment 3

This embodiment explains one example arrangement of TFTs when the analogbuffer circuit of this invention (including embodiment 1 and 2) is usedin an active matrix semiconductor display (typically, liquid crystaldisplay).

FIG. 9 schematically shows a plurality of TFTs that form an activematrix liquid crystal display having the analog buffer circuit of thisinvention. Denoted 901 is a gate signal line side driving circuit havingthe analog buffer of the invention, 902 a source signal line sidedriving circuit, and 903 a pixel matrix circuit. Designated 904, 906 and908 are semiconductor thin films that form TFTs, and 905, 907 and 909are gate electrodes. The shape of the linear laser beam is schematicallyshown as 910. In the figure, the direction of linear laser scanning isshown by arrows.

Although in this embodiment the direction of the active layer, i.e., thedirection of carrier movement, is inclined to the scan direction of thelinear laser (at 45 degrees in this embodiment) as described above, itonly needs to be arranged at inclination and may be set at any otherangle than 45 degrees. In this case, too, the differential circuit B1-Bnare arranged in the same pulse of the linear laser beam and also thecurrent mirror circuits C1-Cn are arranged in the same pulse of thelinear laser beam. It is also possible to arrange both the differentialcircuits B1-Bn and the current mirror circuits C1-Cn in the same pulseof the linear laser beam.

While in the figure gate electrodes are shown for ease of explanation,the actual process may form the gate electrodes after subjecting thesemiconductor film to the laser processing (in the case of top gatetype).

The active matrix liquid crystal display of this embodiment may befabricated using a known manufacturing method. For example, JapanesePatent Application Laid-Open Heisei 8-172049 filed by the applicant ofthis invention offers detailed descriptions. U.S. patent pendingapplication Ser. No. 08/572,104 (filed on Dec. 14, 1995) and Ser. No.09/010,486 (filed on Jan. 21, 1998) correspond to the Japanese patentapplication. The contents disclosed by these Japanese and U.S. patentapplications are incorporated herein by reference.

The laser processing will be briefly explained in the following.

An amorphous silicon film formed over the glass substrate iscrystallized. In this embodiment, the crystalline silicon film that wascrystallized by heating is further subjected to irradiation of a linearlaser beam to enhance the crystallinity.

The laser used for the manufacture of the active matrix liquid crystaldisplay of this embodiment is a KrF excimer laser (wavelength: 248 nm,pulse width: 30 ns). It is possible to use other excimer lasers or othertypes of laser.

The laser beam is oscillated by an oscillator and sent via a totalreflection mirror to an amplifier for amplification, from which it isintroduced into an optical system via a total reflection mirror.

The beam pattern of the laser beam immediately before entering into theoptical system is rectangular, about 3×2 cm². By passing this laser beamthrough the optical system, an elongate beam (linear beam) of about10-30 cm long and about 0.1-1 cm wide can be obtained. The laser beamthat has passed through the optical system has an energy of up to about1000 mJ/shot.

By applying the processed laser beam to the substrate formed with asemiconductor thin film and by moving the substrate in one direction, alaser beam can be irradiated against the entire surface of thesubstrate. A stage that mounts the substrate to be irradiated with thelaser beam is controlled by a computer to move at a desired speed in adirection perpendicular to the linear laser beam. In this way, thesemiconductor thin film is crystallized.

As the number of pixels increases, the current capacity required of theanalog buffer changes. In response to this change, how many smalleranalog buffers should be used to construct each analog buffer alsochanges.

Embodiment 4

The active matrix liquid crystal display devices using the analog bufferof the invention, described in the embodiments 1 to 3, can deal withhigh precision, high resolution active matrix liquid crystal displaydevices, and the number of pixels can be increased to a level whichcorresponds to a future ATV (advanced TV). Therefore, this display canbe used for the active matrix liquid crystal display having a resolutionhigher than XGA, such as 1920 horizontal×1280 vertical.

Embodiment 5

The active matrix liquid crystal displays described in the precedingembodiments 1 to 4 can be used for a transmission type active matrixliquid crystal display and also for a reflection type active matrixliquid crystal display. It is also possible to use as the liquid crystalmaterial a thresholdless antiferroelectric liquid crystal. By usingferroelectric liquid crystal for the liquid crystal material and byusing a special alignment film or by mixing a polymer in the liquidcrystal layer, it is also possible to deal with a case where the memoryeffect of the ferroelectric liquid crystal is erased.

For example, it is possible to use liquid crystal materials disclosed in1998, SID, Characteristics and Driving Scheme of Polymer-StabilizedMonostable FLCD Exhibiting Fast Response Time and High Contrast Ratiowith Gray-Scale Capability by H. Furue et al., 1997, SID DIGEST, 841, AFull-Color Thresholdless Antiferroelectric LCD Exhibiting Wide ViewingAngle with Fast Response Time by T. Yoshida et al., or U.S. Pat. No.5,594,569. The contents disclosed in these papers and patent areincorporated herein by reference.

Particularly, among thresholdless antiferroelectric liquid crystals andthresholdless antiferroelectric mixed liquid crystals made by mixing aferroelectric liquid crystal material and an antiferroelectric liquidcrystal material, there have been found liquid crystal materials whosedrive electric potential is about ±2.5 V. The use of such thresholdlessantiferroelectric mixed liquid crystals with low drive electricpotentials enables the supply electric potential for an image signalsampling circuit to be kept in the range of around 5V to 8V. This isalso effective when TFTs used have a relatively narrow width of LDDregion (low concentration impurity region) (for example, 0-500 nm or0-200 nm).

Here, a graph representing a light transmittivity characteristic of athresholdless antiferroelectric mixed liquid crystal for the appliedelectric potential is shown in a figure. The transmission axis of thepolarizer on the light-incoming side of the liquid crystal display isset almost parallel to the direction of a normal of the smectic layer ofthe thresholdless antiferroelectric mixed liquid crystal, in which thedirection of a normal almost coinciding with the rubbing direction ofthe liquid crystal display. The transmission axis of the polarizer onthe light exit side is set almost perpendicular to the transmission axisof the polarizer on the light-incoming side (a crossed-Nicolsarrangement). It is understood that the use of a thresholdlessantiferroelectric mixed liquid crystal allows a gray-scalerepresentation with the applied electric potential-transmittivitycharacteristic as shown in the graph.

The thresholdless antiferroelectric mixed liquid crystals generally havea high level of spontaneous polarization and therefore a high dielectricconstant. Hence, when the thresholdless antiferroelectric mixed liquidcrystals are used in the liquid crystal display, the pixels are requiredto have a relatively large storage capacitor. It is therefore preferableto use a thresholdless antiferroelectric mixed liquid crystal with smallspontaneous polarization. By using a sequential line driving method forthe liquid crystal display, the period in which to write a gray-scaleelectric potential into the pixel (pixel feed period) can be elongatedand thereby compensate for a small storage capacitor.

The use of the thresholdless antiferroelectric liquid crystals realizesa low electric potential driving and therefore reduces power consumptionof the liquid crystal display.

While the embodiments 1 to 4 have described a case where a liquidcrystal is used as a display medium, any other display medium can beused whose optical characteristics are modulated by applied electricpotentials. For example, electroluminescent elements and electrochromicselements may be used as the display medium.

TFTs used in the embodiments 1 to 5 may be of top gate type or reversestagger type.

Embodiment 6

The semiconductor displays of the embodiments 1 to 5 have a variety ofuses. In this embodiment, semiconductor apparatus incorporating theactive matrix semiconductor display will be explained.

Such semiconductor apparatus include video cameras, still cameras,projectors, head-mounted displays, car navigators, personal computersand portable information terminals (mobile computers and cellularphones). One such example is shown in FIG. 10.

FIG. 10A shows a cellular phone, which includes a body 1001, a voiceoutput section 1002, a voice input section 1003, a semiconductor display1004, an operation switch 1005, and an antenna 1006.

FIG. 10B shows a video camera, which includes a body 1007, asemiconductor display 1008, a voice input section 1009, an operationswitch 1010, a battery 1011, and a picture receiving section 1012.

FIG. 10C shows a mobile computer, which includes a body 1013, a camerasection 1014, a picture receiving section 1015, an operation switch1016, and a semiconductor display 1017.

FIG. 10D shows a head-mounted display, which includes a body 1018, asemiconductor display 1019, and a band portion 1020.

FIG. 10E shows a rear type projector, which includes a body 1021, alight source 1022, a semiconductor display 1023, a polarizing beamsplitter 1024, reflectors 1025 and 1026, and a screen 1027. The reartype projector should preferably be able to change the angle of thescreen with the body fixed according to the viewing position ofaudiences. Three semiconductor displays 1023 (corresponding to R, G andB beams respectively) can be used to realize a rear type projector withhigher resolution and precision.

FIG. 10F shows a front type projector, which includes a body 1028, alight source 1029, a semiconductor display 1030, an optical system 1031,and a screen 1032. Three semiconductor displays 1030 (corresponding toR, G and B beams respectively) can be used to realize a front typeprojector with higher resolution and precision.

Embodiment 7

This embodiment discloses other electronic apparatus utilizing displaysimplementing the present invention. Other electronic apparatus includedigital cameras, head-mounted displays (goggle type display), andpersonal computers (notebook computers). Examples are shown in FIGS. 15Ato 15C.

FIG. 15A shows a personal computer, which includes a body 2001, an imageinput section 2002, a display 2003, and a keyboard 2004. The presentinvention can be applied to the image input section 2002, display 2003and signal control circuits.

FIG. 15B shows a goggle type display, which includes a body 2301, adisplay 2302, and an arm section 2303. The invention can be applied tothe display 2302 and other signal control circuits.

FIG. 15C shows a player that uses a recording medium that has recorded aprogram (simply referred to as a recording medium). The player includesa body 2401, a display 2402, a speaker section 2403, a recording medium2404, and an operation switch 2405. This apparatus can use DVD (digitalversatile disc) and CD as a recording medium to play music, movies andgames and access the Internet. The invention can be applied to thedisplay 2402 and signal control circuits.

FIG. 15D is a digital camera, which includes a body 2501, a display2502, a viewfinder section 2503, an operation switch 2504, and a picturereceiving section (not shown). The invention can be applied to thedisplay 2502 and signal control circuits.

The invention can minimize characteristic variations of the analogbuffer, one of the important causes for image unevenness in the activematrix semiconductor display, and can realize an active matrixsemiconductor display with high image quality.

1. A semiconductor device comprising: n analog buffer circuits directlyconnected in parallel, wherein n is a natural number equal to or greaterthan 2; each of the n analog buffer circuits having a differentialcircuit and a current mirror circuit, wherein the differential circuitcomprising a first thin film transistor and a second thin filmtransistor, wherein one of source and drain of the first thin filmtransistor is electrically connected to one of source and drain of thesecond thin film transistor, and wherein the other one of source anddrain of the second thin film transistor is directly connected to a gateelectrode of the second thin film transistor, wherein the current mirrorcircuit comprising a third thin film transistor and a fourth thin filmtransistor, wherein one of source and drain of the third thin filmtransistor is electrically connected to one of source and drain of thefourth thin film transistor, and wherein the other one of source anddrain of the third thin film transistor is electrically connected to agate electrode of the third thin film transistor, wherein the n analogbuffer circuits are inputted a signal, simultaneously.
 2. Asemiconductor device according to claim 1, wherein the first and secondthin film transistors are P channel thin film transistors, and the thirdand fourth thin film transistors are N channel thin film transistors. 3.A semiconductor device according to claim 1, wherein the first andsecond thin film transistors are N channel thin film transistors, andthe third and fourth thin film transistors are P channel thin filmtransistors.
 4. A semiconductor device according to claim 1, whereineach of the first, second, third and fourth thin film transistors has achannel region comprising polycrystalline.
 5. A semiconductor deviceaccording to claim 1, wherein the semiconductor device is applied to anelectronic apparatus selected from the group consisting of a cellularphone, a video camera, a mobile computer, a head-mounted display, a reartype projector, a front type projector, a player that uses a recordingmedium that has recorded a program and a digital camera.
 6. Asemiconductor device comprising: n analog buffer circuits directlyconnected in parallel, wherein n is a natural number equal to or greaterthan 2; each of the n analog buffer circuits having a differentialcircuit and a current mirror circuit, wherein the differential circuitcomprising a first thin film transistor and a second thin filmtransistor, wherein one of source and drain of the first thin filmtransistor is electrically connected to one of source and drain of thesecond thin film transistor, wherein the other one of source and drainof the second thin film transistor is directly connected to a gateelectrode of the second thin film transistor, and wherein a gateelectrode of the first thin film transistor is electrically connected toan input terminal, and the gate electrode of the second thin filmtransistor is electrically connected to an output terminal, wherein thecurrent mirror circuit comprising a third thin film transistor and afourth thin film transistor, wherein one of source and drain of thethird thin film transistor is electrically connected to one of sourceand drain of the fourth thin film transistor, and wherein the other oneof source and drain of the third thin film transistor is electricallyconnected to the gate electrode of the third thin film transistor,wherein the n analog buffer circuits are inputted a signal,simultaneously.
 7. A semiconductor device according to claim 6, whereinthe first and second thin film transistors are P channel thin filmtransistors, and the third and fourth thin film transistors are Nchannel thin film transistors.
 8. A semiconductor device according toclaim 6, wherein the first and second thin film transistors are Nchannel thin film transistors, and the third and fourth thin filmtransistors are P channel thin film transistors.
 9. A semiconductordevice according to claim 6, wherein each of the first, second, thirdand fourth thin film transistors has a channel region comprisingpolycrystalline.
 10. A semiconductor device according to claim 6,wherein the semiconductor device is applied to an electronic apparatusselected from the group consisting of a cellular phone, a video camera,a mobile computer, a head-mounted display, a rear type projector, afront type projector, a player that uses a recording medium that hasrecorded a program and a digital camera.
 11. A semiconductor devicecomprising: n analog buffer circuits directly connected in parallel,wherein n is a natural number equal to or greater than 2; each of the nanalog buffer circuits having a differential circuit and a current minorcircuit, wherein the differential circuit comprising a first thin filmtransistor and a second thin film transistor, wherein one of source anddrain of the first thin film transistor is electrically connected to oneof source and drain of the second thin film transistor, and wherein theother one of source and drain of the second thin film transistor isdirectly connected to a gate electrode of the second thin filmtransistor, wherein the current mirror circuit comprising a third thinfilm transistor and a fourth thin film transistor, wherein one of sourceand drain of the third thin film transistor is electrically connected toone of source and drain of the fourth thin film transistor, and whereinthe other one of source and drain of the third thin film transistor iselectrically connected to a gate electrode of the third thin filmtransistor, wherein the other one of source and drain of the firsttransistor is electrically connected to the other one of source anddrain of the third transistor, wherein the other one of source and drainof the second transistor is electrically connected to the other one ofsource and drain of the fourth transistor, wherein the n analog buffercircuits are inputted a signal, simultaneously.
 12. A semiconductordevice according to claim 11, wherein the first and second thin filmtransistors are P channel thin film transistors, and the third andfourth thin film transistors are N channel thin film transistors.
 13. Asemiconductor device according to claim 11, wherein the first and secondthin film transistors are N channel thin film transistors, and the thirdand fourth thin film transistors are P channel thin film transistors.14. A semiconductor device according to claim 11, wherein each of thefirst, second, third and fourth thin film transistors has a channelregion comprising polycrystalline.
 15. A semiconductor device accordingto claim 11, wherein the semiconductor device is applied to anelectronic apparatus selected from the group consisting of a cellularphone, a video camera, a mobile computer, a head-mounted display, a reartype projector, a front type projector, a player that uses a recordingmedium that has recorded a program and a digital camera.
 16. Asemiconductor device comprising: a pixel matrix circuit; a source signalline side driving circuit; and a gate signal line side driving circuit;n analog buffer circuits directly connected in parallel in the sourcesignal line side driving circuit, wherein n is a natural number equal toor greater than 2; each of the n analog buffer circuits having adifferential circuit and a current mirror circuit, wherein thedifferential circuit comprising a first thin film transistor and asecond thin film transistor, wherein one of source and drain of thefirst thin film transistor is electrically connected to one of sourceand drain of the second thin film transistor, and wherein the other oneof source and drain of the second thin film transistor is directlyconnected to a gate electrode of the second thin film transistor,wherein the current mirror circuit comprising a third thin filmtransistor and a fourth thin film transistor, wherein one of source anddrain of the third thin film transistor is electrically connected to oneof source and drain of the fourth thin film transistor, and wherein theother one of source and drain of the third thin film transistor iselectrically connected to a gate electrode of the third thin filmtransistor, wherein the n analog buffer circuits are inputted a signal,simultaneously.
 17. A semiconductor device according to claim 16,wherein the first and second tin film transistors are P channel thinfilm transistors, and the third and fourth thin film transistors are Nchannel thin film transistors.
 18. A semiconductor device according toclaim 16, wherein the first and second thin film transistors are Nchannel thin film transistors, and the third and fourth thin filmtransistors are P channel thin film transistors.
 19. A semiconductordevice according to claim 16, wherein each of the first, second, thirdand fourth thin film transistors has a channel region comprisingpolycrystalline.
 20. A semiconductor device according to claim 16,wherein the semiconductor device is applied to an electronic apparatusselected from the group consisting of a cellular phone, a video camera,a mobile computer, a head-mounted display, a rear type projector, afront type projector, a player that uses a recording medium that hasrecorded a program and a digital camera.